1. Field of the Invention
The present invention relates to a cathode ray tube which optimizes a radius of curvature in the directions of a long axis, a short axis and a diagonal axis of a shadow mask as the deflection angle of an electron beam is increased.
2. Description of the Background Art
FIG. 1 is a sectional view illustrating an internal structure of a general cathode ray tube.
In general, a cathode ray tube comprises a vacuum envelope 1 having a panel 2 and a funnel 4 coupled to each other.
In the vacuum envelope 1, included are a screen 3 having a plurality of phosphors coated thereon, an electron gun (not shown) for emitting an electron beam to make the phosphors give off light by hitting the phosphors of the screen 3, and a shadow mask 6 for discriminating the electron beam in colors so that the electron beam emitted toward the screen 3 from the electron gun can hit the phosphors of the screen 3.
The shadow mask 6 includes a plurality of electron beam through holes formed in the effective area through which an electron beam passes.
Besides, on the outer side of the vacuum envelope 1, included is a deflection yoke 8 for deflecting the electron beam vertically and horizontally so that the electron beam emitted toward the screen 3 from the electron gun can reach the screen 3 through any one of the plurality of electron beam through holes formed on the shadow mask 6.
In the thus-constructed cathode ray tube, the electron beam emitted from the electron gun is horizontally and vertically deflected by the deflection yoke 8 to advance, and reaches the screen 3 through the electron beam through holes of the shadow mask 6, whereby the phosphors coated on the screen 3 give off light by the electron beam that has reached the screen 3 to realize an image.
Meanwhile, in recent years, cathode ray tubes have become larger, flatter and slimmer as the strategies for acquiring a competitive edge over other display manes such as LCDs and PDPs. Especially, the cathode ray tube inevitably has a more disadvantageous structure compared to the other display means in terms of getting slimmer due to the characteristics of products that achieve a screen image by projecting an electron beam.
In other words, in order for the cathode ray tube to become larger and flatter, the deflection angle of the electron beam has to be increased as compared to the conventional art. The increase of the deflection angel of the electron beam means that the radius of curvature of the inner surface of the panel 2 needs to become flatter and accordingly, the shadow mask 6 needs to be flatter too.
However, if the shadow mask 6 becomes flatter, the structural strength of the shadow mask 6 becomes weaker, which leads to a problem that thermal distortion, vibration characteristics and drop characteristics are greatly deteriorated. Thus, a design for optimizing the radius of curvature of the shadow mask 6 is required.
Further, if the radius of curvature of the end of an effective surface of the shadow mask 6 is small, the structural strength of the shadow mask 6 can be enhanced, while an inner pin distortion occurs in which a screen image appears to be bent inward of the screen 3 as seen in the region indicated by ‘A’ in FIG. 2. The inner pin distortion becomes more serious when the radius of curvature of the shadow mask 6 sharply decreases toward the peripheral portions of the shadow mask. Therefore, a design for optimizing the radius of curvature of the shadow mask 6 considering the aforementioned inner pin distortion is required.
That is to say, if the radius of curvature of the end of the effective area of the shadow mask 6 is decreased, the structural strength of the shadow mask 6 can be improved but a screen distortion caused by the inner pin distortion gets worse.
Moreover, if the shadow mask 6 is formed with a single radius of curvature so that it becomes flat, the screen distortion caused by the inner pin distortion is improved, but a thermal distortion occurs due to a decrease of the structural strength of the shadow mask 6 and as a result, problems caused by doming and vibration characteristics are generated.